The need for sometimes removing objects which have become tightly lodged in oil wells has been known for a long time. Such objects include, inter alia stuck drill bits, liners, casing, and pumps.
One commonly used technique has involved pulling on the drill-string with a static or slowly varying force, in order to overcome the static friction or tractive force holding the particular object in the drill hole. Many tools can be used to develop large static tensional loads in the drill string, usually in conjunction with the oil well derrick. Unfortunately, the purely static technique often requires forces so great as to induce yielding, or even failure, in the drill string when trying to free the stuck object. If the safe working stress has been exceeded, then the entire drill string must be removed and inspected, and possibly replaced, at great expense and loss of drilling production time.
A second technique employs a vibrator to excite vibratory motion in the drill string (and attached stuck object) at or near one of its longitudinal resonant frequencies while pulling upward on the string. Examples of this technique are shown in Bodine's U.S. Pat. Nos. 2,972,380 and 4,236,580.
The Bodine patents appear to be somewhat theoretical, since no specific examples are given. The frequencies appear to be thought of as within the sonic range. The vibrations are generally produced by rotary eccentric masses, sometimes driven by hydraulic motors.
Resonant dynamic excitation does offer significant advantages over the simple static-force approach to removal of lodged objects:
(1) In the case of adhesion to the hole, small cyclic movement can, as the Bodine patents point out, heat the adhesive material (e.g., tars or other long-chain polymers), thus decreasing its viscosity and, hence, the tractive force to be overcome;
(2) cyclic longitudinal force in the drill string was known by Bodine to produce a varying radial expansion and contraction in the string, which helps to overcome the traction;
(3) the cyclic force, as I have recently discovered can be concentrated in the uppermost part of the stuck object, thereby increasing the local stress needed to overcome traction. Thereby, the use of resonant dynamic loading in conjunction with static tension can lower the total stress required to free the stuck object. This, in turn, can reduce the likelihood of damaging the drill string, while improving the likelihood of successfully removing the stuck object.
In a system which is controlled by the power input (e.g., the rotational speed of an engine), a potential "runaway" situation exists, for when the maximum power input for a particular resonance is exceeded, the engine may speed up greatly because the pipe can absorb less power at a frequency higher than resonance. This problem will be explained below in more detail.
Another potential problem is that of exciting harmful modes of vibration of the derrick. Modes of vibration which have a lower resonant frequency than the desired mode and which involve different parts of the derrick and support structure, have large and potentially harmful vibrational amplitudes. A system which increases the operating frequency to arrive at the desired mode tends to excite these harmful modes and create hazardous conditions.
Among the objects of the invention are these: to provide a practical and economic method for driving the drill pipe or string at resonance; to keep it at resonance when and if the resonant frequency changes; to provide for relatively low power operation; to provide controls that protect the apparatus from damaging itself; and to provide for pulses, when needed, to aid freeing the stuck object.